The fans of "Supernatural" were in for a huge surprise in season 11 episode 10 "The Devil in the Details", when Lucifer (Mark Pellegrino), having been denied permission to enter Sam's (Jared Padalecki) body multiple times, got the green signal from Castiel (Misha Collins) to use his body. In the upcoming episode 11, the consequences of Castiel's decision will be revealed.

In "The Devil in the Details" it was revealed that Rowena (Ruth Connell) had conducted a spell that allowed Lucifer to jump bodies. Unbeknownst to the Winchesters, Castiel's body now has a new occupant – the Devil. In the promo for upcoming "Into the Mystic" Dean (Jesen Ackles) is seen telling his brother that there is something "off about him".

Meanwhile, Castiel aka Lucifer is seen turning angels into dusk just with the snap of his fingers. The only reason Cas made the deal with Lucifer is because the fallen angel promised him that he would destroy Amara aka Darkness (Emily Swallow). Will the Devil hold up his end of the deal?

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If he does take up the battle, we will get to see an episode of the season 5 finale "Swan Song" magnitude. In "Swan Song" Lucifer – then possessing Sam -- and Michael (Jake Abel) almost fought each other that would have resulted in the Apocalypse predicted by Bible. However, Sam, Dean and Cas stepped in at the last moment and saved the whole world.

If Lucifer fulfils his promise to Cas, he will technically fight his aunt, aka God's sister Amara in season 11. While Lucifer vs Darkness is a fight every "Supernatural" fan would enjoy, it is hard to understand Cas' reasoning behind working with the Devil.

As Sam cleverly questioned in "Devil in the Details", what would Lucifer do, once he defeats Darkness? Try to herald the Apocalypse again? It also makes things a little confusing. Can Angels possess the body already occupied by another angel without seeking permission from the human? As fans know Jimmy is already dead, and Castiel has been using his body as his suit since he died, so for Lucifer to take over, did Castiel have to agree to be in the cage?

Find out in season 11 episode 11 of "Supernatural" which will be aired at 9pm (EST) on Wednesday, 27 January. You can also live stream "Into the Mystic" via The CW website.